With a massive, "gaudy" ring on his left hand, Justin Theroux attended HBO's Emmys bash without his significant other Jen Aniston.Credit: Jeff Kravitz/FilmMagic

What's up with the ring?! Justin Theroux wore a massive ring on that finger while attending HBO's official Emmys afterparty — without fiancee Jennifer Aniston — on Monday, Aug. 25, at the Pacific Design Center in Hollywood.

An observer inside the bash note to Us Weekly that The Leftovers hunk, 43, "appeared to be in a great mood," even without his mega-famous significant other by his side.

Still, solo Theroux, in the spotlight lately for his intense Leftovers performance, was "jovial," "friendly," and happy to run into pals like Julianna Margulies and Sarah Paulson, a second eyewitness said. According to one observer, Best Actress in a Drama winner Margulies and Theroux even broke it down on the dance floor when the DJ played "Mr. Saxobeat."

Added the insider, "It was weird that Jen didn't come, because it seemed like Justin was having a great time catching up with friends." The actor and screenwriter, who was the main focus at the event, even "held court at a table in the center of the party."

Bigger than Theroux, though, was the ring he was wearing on his left hand. An observer described it as a "big gold class ring," which observers immediately noticed. "It was clearly not a wedding band," one insider noted, while another described it as a "large gaudy ring." (Theroux has been spotted wearing the ring on previous occasions.)

Though Aniston didn't accompany Theroux to any Emmys events on Monday evening, she accepted his ALS Ice Bucket Challenge this past weekend. (He even dumped freezing water over her head!)

The pair have also gushed about each other in recent months, updating fans on the status of their engagement. "He's just so beautiful and handsome to me," Aniston told Women's Wear Daily last month of her beau. "I love that his eyeballs are so beautifully captured because those eyes just knock me out every day… He's just always been there and been brilliant, and now this is just in a different light." See more cute pics of Jen and Justin here.